Dark streaks on Mars bolster case for liquid water

Mars’s dust bowl image may need a makeover. Dark streaks seen forming in summer and fading in winter might be signs of water flowing just beneath the surface.

The sudden appearance of streaks on sloping ground have been attributed to present-day liquid water before, although their origin is still debated. Light streaks have been seen appearing on steep slopes in images taken years apart by NASA’s Mars Global Surveyor spacecraft. And seasonal dark streaks have emerged in the north polar region.

Liquid water flowing downhill might explain both types of events, but dust or sand avalanches could also be to blame.

Advertisement

Now, new images have revealed a previously unknown population of seasonal dark streaks in Mars’s southern hemisphere, with characteristics that seem to tie them to liquid water.

Salty antifreeze

Researchers led by Alfred McEwen of the University of Arizona, Tucson, found slopes where dark streaks appear every summer and disappear each winter in images from NASA’s Mars Reconnaissance Orbiter (MRO).

They have been seen mainly on slopes in the southern hemisphere at mid-latitudes, though some have been seen near the equator, as well.

Since they appear in the summer when the ground is warmest, one possibility is that they result from ice that melts and drains down slopes. The researchers believe any flowing water lies below the surface because if it were above it, MRO probably would have spotted its spectral signature.

Some of the streaks form when temperatures are only -23 °C, well below the freezing point of pure water. But salty water can remain liquid at such temperatures, possibly explaining the streaks.

Shifting grains

The researchers think salty water flowing just beneath the surface might shift dust grains up above, causing the dark streaks. However, the explanation is not watertight – it is not clear why the streaks would then disappear in colder weather.

“The best explanation we have for these observations so far is flow of briny water, although this study does not prove that,” says McEwen.

The seasonal streaks, which the team calls recurring slope lineae, show no preference for dusty areas, where dust avalanches would be more likely.

However, the streaks do occur in the same zone of the southern hemisphere – between 30 and 60 degrees latitude – where there is previous evidence for buried ice, including what appear to be glaciers buried under rock.

Toehold for life?

The ice deposits may have formed when past climate swings deposited snow on the planet’s mid-latitudes.

“They’ve added to the case for subsurface ice and salts in the mid-latitudes, which, not too surprisingly, can melt and flow short distances downhill,” says Phil Christensen of Arizona State University in Tempe, who was not involved in the study.

“The correlations with known deposits of snow and ice look very strong,” agrees James Head of Brown University in Providence, Rhode Island, who was also not involved in the study.

The discovery of what might be liquid water on present-day Mars raises the possibility that life may have a toehold there. Keeping water liquid at very low temperatures requires very salty water, in which life as we know it would have difficulty surviving, however.

But researchers are not ruling out the possibility. “This is a very exciting discovery because it is our first chance to see an environment on Mars that might allow for the expression of an active biological process if there is present-day life on Mars,” says Lisa Pratt of Indiana University in Bloomington, who was not involved in the study.